We present deep H I 21 cm and optical observations of the face-on spiral
galaxy M 83 obtained as part of a project to search for high-velocity
clouds (HVCs) in nearby galaxies. Anomalous-velocity neutral gas is
detected toward M 83, with 5.6 × 107 M sun
of H I contained in a disk rotating 40-50 km s-1 more
slowly in projection than the bulk of the gas. We interpret this as a
vertically extended thick disk of neutral material, containing 5.5% of
the total H I within the central 8 kpc. Using an automated source
detection algorithm to search for small-scale H I emission features, we
find eight distinct, anomalous-velocity H I clouds with masses ranging
from 7 × 105 to 1.5 × 107 M
sun and velocities differing by up to 200 km
s-1 compared to the H I disk. Large on-disk structures
are coincident with the optical spiral arms, while unresolved off-disk
clouds contain no diffuse optical emission down to a limit of 27 r' mag
per square arcsec. The diversity of the thick H I disk and larger clouds
suggests the influence of multiple formation mechanisms, with a galactic
fountain responsible for the slowly rotating disk and on-disk discrete
clouds, and tidal effects responsible for off-disk cloud production. The
mass and kinetic energy of the H I clouds are consistent with the mass
exchange rate predicted by the galactic fountain model. If the HVC
population in M 83 is similar to that in our own Galaxy, then the
Galactic HVCs must be distributed within a radius of less than 25 kpc.